I respectfully disagree with your first line. The conventional way to express base 26 is using the letters A thru Z, not starting with 0123etc. as you suggest. I agree that hexadecimal uses both numbers and letters as you show.
But ok, then in any base where the letter O is included, then
x = O - 1 = N (and it stays in whatever base one chooses).
re(2): Another Try at a Solution - Spoiler
